iPhoto '11: If you see a message about rebuilding your thumbnail cache

If you see a message about rebuilding your thumbnail cache

Over time, iPhoto can become less efficient at accessing the photos in your library. Rebuilding the cache can help iPhoto display your photos faster, and can also resolve issues such as thumbnails that appear to be dimmed or missing.

After upgrading iPhoto, you might see a message asking if you want to rebuild your thumbnail cache. You can choose whether to do it then or come back to it later.